Describe how to configure a custom event trigger that fires only when a specific parameter within a pushed data layer event matches a particular regular expression.

To configure a custom event trigger in Google Tag Manager (GTM) that fires only when a specific parameter within a pushed data layer event matches a particular regular expression, follow these steps: First, create a new trigger in GTM and select 'Custom Event' as the trigger type. Enter the name of the event that is pushed to the data layer. This is the 'event' value in the `dataLayer.push()` call. For example, if the data layer event is `dataLayer.push({'event': 'formSubmit', 'formID': 'contactForm'})`, you would enter 'formSubmit' as the event name. Next, add a condition to the trigger that checks if the specific parameter matches the regular expression. Click 'Add Filter' to add a new condition. Select a ....